As part of an experimental study on the effects of jigsaw learning on Vietnamese tertiary students' longer term achievement in a course of Management of Education and Administration, knowledge retention after one month was investigated. As reported in our previous papers, students in the cooperative jigsaw group (N = 40), perceived their instruction as more cooperative and more student-centered, and less teacher-centered than did those in the lecture group (N = 40). They also obtained significantly higher scores on a post experiment achievement test than did the others. Results indicate that in general students in the cooperative jigsaw group appreciated most working with others and getting help, discussing and sharing information and teaching others, and enjoyed the jigsaw context. This paper furthers that analysis by investigating students' knowledge retention. The findings show that a month after the experiment, students in the jigsaw group had greater long-term achievement than those did in the lecture group. However, students in both groups had a similar percentage of knowledge retention on the delay test of achievement. They retained nearly all of what they had learned (approximately 99 percent of knowledge) a month after the six weeks of the instruction.